Since the invention of integrated circuits, the number of devices on a chip has grown at a near-exponential rate. The fabrication methods of the semiconductor industry have been modified and improved continuously for almost four decades. With each improved method, the capacity of a single semiconductor chip has increased from several thousand devices to hundreds of million devices. Future improvements will require integrated circuit devices such as transistors, capacitors, and connections between devices to become even smaller and more densely populated on the chip.
The increased packing density of the integrated circuit generates numerous challenges to the semiconductor manufacturing process. Every device must be smaller without damaging the operating characteristics of the integrated circuit devices. High packing density, low heat generation, and low power consumption, with good reliability and long operation life must be maintained without any functional device degradation. Increased packing density of integrated circuits is usually accompanied by smaller feature size.
As integrated circuits become denser, the widths of interconnect layers that connect transistors and other semiconductor devices of the integrated circuit are reduced. As the widths of interconnect layers and semiconductor devices decrease, their resistance increases. As a result, semiconductor manufacturers seek to create smaller and faster devices by using, for example, a copper interconnect instead of a traditional aluminum interconnect. Unfortunately, copper is very difficult to etch in a semiconductor process flow. Therefore, damascene processes have been proposed to form copper interconnects.
The damascene method involves forming a trench and/or an opening in a dielectric layer that lies beneath and on either side of the copper-containing structures. Once the trenches and/or openings are formed, a blanket layer of the copper-containing material is formed over the entire device. Electrochemical deposition (ECD) is typically the only practical method to form a blanket layer of copper. The thickness of such a layer must be at least as thick as the deepest trench and/or opening. After the trenches and/or the openings are filled with the copper-containing material, the copper-containing material over the trenches/openings is removed, e.g., by chemical-mechanical polishing (CMP), so as to leave the copper-containing material in the trenches and openings but not over the dielectric or over the uppermost portion of the trench/opening.
Copper deposited by ECD, however, has fine grains and will re-crystallize during subsequent processing steps. During anneal steps, deposited copper interconnects frequently form voids at via bottoms and other interfaces, which may ultimately cause device failure. One solution is to anneal the copper at low (below 200° C.) temperatures. A low temperature anneal, however, will not completely stabilize the deposited copper and also result in device failure. Finally, the copper may be annealed at high temperatures. Stresses within the copper interconnect structure may cause the interconnect to fail during the high temperature annealing process, which also causes via open failures.
Copper via and interconnect fabrication processes are growing in use. One example of a conventional interconnect fabrication process is depicted in FIGS. 1A-1E. In particular, FIG. 1A depicts a typical damascene interconnect process in which an interlevel dielectric (ILD) 12 is formed over a semiconductor body 10. The interlevel dielectric 12 is then patterned and etched to remove the dielectric material from the areas 14 where the interconnect lines are desired, as depicted in FIG. 1B. Referring now to FIG. 1C, a barrier layer 16 is then deposited over the structure including over the dielectric 12 and in the areas 14 where the dielectric has been removed. A copper seed layer 18 is then formed over the barrier layer 16. The copper layer 20 is then formed from the seed layer 18 using, for example, electrochemical deposition (ECD), which is also known as an electroplating process, as depicted in FIG. 1D. Chemical-mechanical polishing (CMP) is then used to remove the excess copper and planarize the copper 20 with the top of the interlevel dielectric layer 12, as depicted in FIG. 1E.